Pool professionals rely on the Langelier Saturation Index (LSI) to assess the balance of pool water. This index provides a formula that considers the interplay between several key factors: pH, total alkalinity, calcium hardness, temperature, and total dissolved solids (TDS). It's a go-to method for addressing complex water balance issues, and there are even mobile apps and calculators based on the LSI to assist in this process.

The LSI formula is as follows:

LSI = pH + F(T) + F (TA) + F(CH) – F(TDS)

In this formula, temperature, total alkalinity, calcium hardness, and total dissolved solids each have specific factors assigned, as detailed in the accompanying chart. The pH value is used as it is directly measured from the pool, without an additional factor.

Applying this formula, an LSI calculation of zero signifies water is perfectly balanced. A negative LSI value, particularly less than -0.3, suggests the water is corrosive, while a positive LSI value over +0.5 indicates a tendency toward scale formation. It's generally preferable for the LSI to be slightly positive rather than negative, as minor scale formation offers some protection against the more damaging effects of corrosion.

However, it's important to note that achieving a balanced LSI does not always mean the water is truly balanced, especially if the parameters used are outside recommended levels. For instance, consider a pool with a pH of 8, temperature of 84 °F, calcium hardness of 250 ppm, total carbonate alkalinity of 25 ppm, and TDS of 1,000 ppm. Using the LSI formula, this might seem balanced:

LSI =

+ 0.7 + 2 + 1.4-12.1 = 0

Despite this, the high pH and calcium levels could still create conditions favorable for scale formation. Thus, even with an LSI suggesting balance, this scenario doesn't truly represent balanced water conditions.

Achieving an LSI of zero using values within recommended ranges is the only surefire indicator of genuinely balanced water.